25 August 2014

Cristobal heading north and out to sea

Since my previous post on Saturday afternoon, TD4 was upgraded to Tropical Storm Cristobal on Sunday morning.

Cristobal is the latest third named storm to form since Charlie in 1992 (named on Sep 22).  You may recall from previous posts that 1992 was also the last year that both the A and the B storms became hurricanes.  Well, if Cristobal strengthens into a hurricane (which it could), 1992 is ALSO the last time the A, B, and C storms all became hurricanes!  And yet another similarity between 1992 and 2014 is that the A storm made landfall on the U.S. as a hurricane (though Andrew was a Category 5 and Arthur was a Category 2).

As of Monday morning, Cristobal is drifting north at about 3mph, and is centered just north of Mayaguana in the eastern Bahamas.  The intensity estimate as of 8am EDT is 60mph with a 994mb central pressure.  It is forecast to become a hurricane within the next day or two.

Visible satellite image from 8:15am EDT.  I overlaid the past track in light blue and marked the current center location with a light blue X.  (NASA)
As you can see in the image above, the surface center is exposed, and all of the thunderstorm activity is displaced to the south due to strong northerly wind shear.  The sea surface temperature and ocean heat content are extremely favorable for further intensification, but due to the shear, the NHC intensity forecast currently brings the storm up to a minimal Category 1 hurricane.

The northward turn that was in question for so many days did finally happen on Sunday, and was the favored solution by the majority of models.  There was only a slight chance for a westward track into south Florida, but given the potential impact there, it was worth monitoring that possibility very closely.  The map below shows the surface pressure in line contours and the 500mb heights (steering features) in shaded contours as of this morning... Cristobal will head northeast into the trough (yellow).

Surface pressure and 500mb height contours valid at 8am EDT today.  (tropicaltidbits.com)
Now, models have come into strong agreement on Cristobal's future.  Once it pulls away from the eastern Bahamas, it is forecast to pass west of Bermuda on Wednesday as a hurricane, then zip out into the north central Atlantic where it will become a potent extratropical cyclone by the weekend.

Monday morning's suite of model and consensus guidance.  (UW-Milwaukee)
Elsewhere, there is a weak easterly wave located about 1400 miles east of the Windward Islands, but environmental conditions will inhibit development for the foreseeable future.  This wave can be tracked back to the African coast on August 21.

23 August 2014

Tropical Depression 4 forms north of Haiti

The disturbance we've been watching struggle across the Atlantic for the past 13 days is now north of Haiti... and has just been upgraded to Tropical Depression 4.  Aircraft reconnaissance missions have been carried out frequently, but up until now, it has either been lacking enough centralized thunderstorm activity or a closed circulation that extends all the way to the surface.  Both are requirements for a tropical cyclone (that generic term includes tropical depressions, tropical storms, and hurricanes). 

Because it has remained so poorly organized, models have had a hard time with it.  Stronger cyclones are steered by different layers of the atmosphere than weaker cyclones, so if a model is too bullish on the intensity in the analysis or forecast, it will likely get the track wrong.  For the past several days, this track forecast has revolved around whether or not a weakness in the subtropical ridge is strong enough to allow the disturbance to slide northward into it, or if it will not "feel" that weakness and continue a west-northwest heading.

The difference between those scenarios is rather big: it either hits south Florida or recurves over the Bahamas and stays well away from the US... with some 'wiggle room' in between.  I would still estimate a 10-15% chance of the south Florida option happening, which is noteworthy because it would only be 3 days away and the Bahamas are infamous for producing rapid intensifiers.  The wind shear is low, the SST is very warm, the only obvious obstacle now is its proximity to Hispaniola.

The 12Z suite of "late" models, including the 20 GFS ensemble members.

The eastern Bahamas are now under a tropical storm warning, and the official NHC forecast (with cone of uncertainty) is shown below.  Their forecast brings it up to tropical storm intensity tonight, and then hurricane intensity on Tuesday... right in the middle of the pack in the figure above.  If you're in south Florida, this should not be ignored.  There is a slight possibility that soon-to-be Cristobal could make landfall there as a minimal hurricane in just three days.  If you're in coastal SC or NC, you should also be paying very close attention to this in the coming days.

21 August 2014

Tropical disturbance slowly organizing, model tracks shift

Since yesterday's post, the area of interest east of the Lesser Antilles has gotten slightly better organized, and more interestingly, the models have all shifted their forecasts north.  Keep in mind that this is not yet a tropical depression, so there is tremendous uncertainty in any forecast you see (hard to forecast "it", when "it" isn't even a tropical cyclone).  And even with a bona fide tropical cyclone, model forecasts beyond five days contain a lot of uncertainty.

Visible satellite image from 8:15am EDT.  I added a red L to help locate the center of the disorganized disturbance.  (NRLMRY)
The northwestward motion that we've been seeing is due to a weakness in the subtropical ridge... or in other words, rather than being pushed west or west-northwest like storms typically would in this location, it's moving north much quicker than usual.  It is forecast to pass over the northern Leeward Islands early Friday morning, but shouldn't be more than a gusty and wet day there.

Though too complicated to explain in a caption, the steering feature I'm describing can be found in the area north of Puerto Rico and Hispaniola.  The line contours are the current 500mb heights, and the shaded contours are the height anomalies. (tropicaltidbits.com)
As far as the latest model forecasts go, a storm entering the Gulf of Mexico now looks like a long-shot, whereas yesterday it looked relatively likely.  Models  seem to have a better grip on the structure of that subtropical ridge and show a northward turn sometime around Sunday when the "storm" is near Hispaniola.  BUT, any change in the strength of that ridge will make a huge difference in track... if it builds, the storm would start moving more west, and if it weakens further, the storm will nudge further north.

Tracks from the 06Z model runs... the first four are global models and the last four are regional hurricane models. (U.Albany)

Of historical note, today is the 7-year anniversary of Hurricane Dean's landfall on Mexico's Yucatan peninsula.  Dean hit near Chetumal with sustained winds estimated at 170+ mph.  It remains the strongest landfalling hurricane in the Atlantic since Andrew hit south Florida in 1992.

Enhanced infrared satellite image of Hurricane Dean making landfall during the early morning hours of August 21, 2007. (NRLMRY)

07 August 2014

Hurricane Iselle to make landfall on Hawaii tonight

I don't normally write about tropical cyclones that aren't in the Atlantic on this blog, but there's a significant event unfolding for Hawaii and I figured others would find it interesting too.

The post is available on the Capital Weather Gang blog:

Record-breaking Hurricane Iselle to make landfall on Hawaii’s Big Island tonight

04 August 2014

Bertha becomes season’s second hurricane

[Edit... 45 minutes after I wrote the post, it was upgraded to a hurricane]

Today's update on Bertha can be found on the Capital Weather Gang blog:

Bertha becomes season’s second hurricane

01 August 2014